This invention relates to a low inductance ground lead for grounding a signal probe used in high frequency measurements.
A common problem in the measurement of high frequency signals is that the self-inductance of the ground connection to the signal probe combines with the input capacitance of the signal probe to form a series resonant circuit that not only reduces the amplitude of the input signal with increasing frequency, but produces ringing. This increasingly degrades the accuracy of the measurement system with increasing signal frequency. Typically, the ground connection to a signal probe comprises a ground lead made of braided wire, and a ground probe to which the ground lead is connected, both of which have relatively high self-inductances.
The accuracy of high frequency measurements (on the order of 100 megahertz and higher) becomes compromised as signal frequencies approach the frequency of the series-resonant circuit formed by the inductance in the signal path and the input capacitance of the signal probe. The resonant frequency f of such a circuit is determined by the following equation: ##EQU1## where L is the inductance of the signal path and C is the capacitance of the signal probe.
The capacitance of typical signal probes in use for high frequency measurements is on the order of 10-15 picofarads. The self-inductance of the signal probe from the signal source to the point of contact with the ground lead, the inductance of the ground lead, and the self-inductance of the ground probe (which is the means of completing the circuit to ground) from the point of contact with the ground lead to ground all contribute to the inductance of the signal path. However, the inductance of the ground lead is the primary contributor to the total inductance in the signal path.
The inductance of conventional ground leads is on the order of 200-300 nanohenries. Given typical signal probe capacitances, the frequency of the series-resonant circuit begins to degradce high frequency measurements as signal frequencies exceed 100 megahertz. The degree of degradation increases as the frequency of the signal increases.
The previous steps taken to alleviate this problem of degrading accuracy have included reducing the self-inductance of the ground connection by reducing the length of the ground lead, using as a ground lead a conductor having a large circular cross-section, and eliminating the ground probe and instead contacting ground directly with the ground lead. However, these steps produce the disadvantage of making the probe inconvenient to use. The limited reach of such a ground lead often makes it difficult to connect to an available ground. A lead with a large circular cross-section is relatively inflexible, which makes it difficult to connect to, and keep in contact with ground. Moreover, frequent bending of such a lead results in early breakage due to work hardening of the metal. And, elimination of the ground probe makes the ground lead difficult to manipulate and continuity of the ground connection difficult to maintain.
Another approach to alleviating the aforementioned problem of ground probe self-inductance has been to connect a short wire to the ground of the signal probe by wrapping one end of the wire around the signal probe near its tip and holding the other end of the wire in contact with ground. While this solution avoids the self-inductance of the signal probe, it does not permit use of a significantly longer ground lead.
Accordingly, there is a need for an improved grounding connection for use with high frequency signal probes that reduces ground connection self-inductance and resultant signal degradation, while permitting convenient and assured, continuous ground.